There has been a rapid growth in the market for linear low density polyethylene (LLDPE), particularly resin made under mild operating conditions; typically at pressures of 100 to 300 psi and reaction temperatures of less than 100.degree. C. This low pressure process provides a broad range of LLDPE products for blown and cast film, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, about 2 to 6 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer used in the polymerization. Although the majority of the LLDPE resins on the market today have a narrow molecular weight distribution, LLDPE resins with a broad molecular weight distribution are available for a number of non-film applications.
LLDPE resins designed for commodity type applications typically incorporate 1-butene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to those of ethylene/1-butene copolymers. The predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 4-methyl-1-pentene, and 1-octene. The bulk of the LLDPE is used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes this film well suited for a broad spectrum of applications. Fabrication of LLDPE film is generally effected by the blown film and slot casting processes. The resulting film is characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance.
These properties together with toughness are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases. By providing a blend of polymers, the properties characteristic of high molecular weight resins can be retained and processability, particularly the extrudability (from the lower molecular weight component) can be improved.
The blending of these polymers is successfully achieved in a staged reactor process similar to those described in U.S. Pat. Nos. 5,047,468 and 5,149,738. Briefly, the process is one for the in situ blending of polymers wherein a higher density ethylene copolymer is prepared in a high melt index reactor and a lower density ethylene copolymer is prepared in a low melt index reactor. The process typically comprises continuously contacting, under polymerization conditions, a mixture of ethylene and one or more alpha-olefins with a catalyst system in two gas phase, fluidized bed reactors connected in series, said catalyst system comprising: (i) a supported magnesium/titanium based catalyst precursor; (ii) an aluminum containing activator compound; and (iii) a hydrocarbyl aluminum cocatalyst, the polymerization conditions being such that an ethylene copolymer having a melt index in the range of about 0.1 to about 1000 grams per 10 minutes is formed in the high melt index reactor and an ethylene copolymer having a melt index in the range of about 0.001 to about 1 gram per 10 minutes is formed in the low melt index reactor, each copolymer having a density of about 0.860 to about 0.965 gram per cubic centimeter and a melt flow ratio in the range of about 22 to about 70, with the provisos that:
(a) the mixture of ethylene copolymer matrix and active catalyst formed in the first reactor in the series is transferred to the second reactor in the series; PA1 (b) other than the active catalyst referred to in proviso (a) and the cocatalyst referred to in proviso (e), no additional catalyst is introduced into the second reactor; PA1 (c) in the high melt index reactor: PA1 (d) in the low melt index reactor: PA1 (e) additional hydrocarbyl aluminum cocatalyst is introduced into the second reactor in an amount sufficient to restore the level of activity of the catalyst transferred from the first reactor to about the initial level of activity in the first reactor. PA1 (a) the precursor is formed by contacting an alkylaluminum halide with a solid reaction product prepared from a magnesium alkoxide, a titanium tetraalkoxide, and a solubility enhancing agent; PA1 (b) the mixture of ethylene copolymer matrix and active catalyst formed in the first reactor in the series is transferred to the second reactor in the series; PA1 (c) other than the active catalyst referred to in proviso (b) and the cocatalyst referred to in proviso (f), no additional catalyst is introduced into the second reactor; PA1 (d) in the first reactor in which a low melt index copolymer is made: PA1 (e) in the second reactor in which a high melt index copolymer is made: PA1 (f) additional hydrocarbyl aluminum cocatalyst is introduced into the second reactor in an amount sufficient to restore the level of activity of the catalyst transferred from the first reactor to about the initial level of activity in the first reactor.
(1) the alpha-olefin is present in a ratio of about 0.02 to about 3.5 moles of alpha-olefin per mole of ethylene; and PA2 (2) hydrogen is present in a ratio of about 0.05 to about 3 moles of hydrogen per mole of combined ethylene and alpha-olefin; PA2 (1) the alpha-olefin is present in a ratio of about 0.02 to about 3.5 moles of alpha-olefin per mole of ethylene; and PA2 (2) hydrogen is, optionally, present in a ratio of about 0.0001 to about 0.5 mole of hydrogen per mole of combined ethylene and alpha-olefin; and PA2 (1) alpha-olefin is present in a ratio of about 0.005 to about 0.4 mole of alpha-olefin per mole of ethylene; PA2 (2) hydrogen is, optionally, present in a ratio of about 0.0001 to about 0.3 mole of hydrogen per mole of ethylene; and PA2 (3) the ethylene partial pressure is in the range of about 10 to about 100 pounds per square inch; and PA2 (1) alpha-olefin is, optionally, present in a ratio of about 0.005 to about 0.6 mole of alpha-olefin per mole of ethylene; PA2 (2) hydrogen is, optionally, present in a ratio of about 1.0 to about 3.0 moles of hydrogen per mole of ethylene; and PA2 (3) the ethylene partial pressure is in the range of about 50 to about 120 pounds per square inch; and
While the in situ blends prepared as above and the films produced therefrom are found to have the advantageous characteristics heretofore mentioned, the commercial application of these granular bimodal polymers is limited by unacceptable levels of gels in contrast to competitive bimodal resins produced from slurry or solution processes. Particle size distribution and flow characteristics studies indicate that the gas phase resins having an average particle size (APS) of about 400 to about 600 microns exhibit significant compositional, molecular, and rheological heterogeneities. When such a granular resin is compounded, for example, with a conventional twin screw mixer in a single pass, and the resulting pellets are fabricated into film, the film exhibits a high level of gels ranging in size from less than about 100 microns to greater than about 500 microns. These gels adversely effect the aesthetic appearance of the product. The gel characteristics of a film product are usually designated by a subjective scale of Film Appearance Rating (FAR) varying from minus 50 (very poor; these films have a large number of large gels) to plus 50 (very good; these films have a small amount of, or essentially no, gels). The FAR of the single pass film product mentioned above is generally in the range of about minus 50 to about minus 10/0. For commercial acceptability, the FAR should be plus 20 or better.
Several patents have successfully addressed the gel problem. One such patent is U.S. Pat. No. 5,405,901. Unfortunately, this patent, for example, suffers from other deficiencies. The extrusion line speed of the product resin, which is a measure of bubble strength and stability, is found to be in the neighborhood of 90 feet per minute, and the dart drop strength, while high for one mil film, is not achievable on 0.5 mil film due to low bubble stability. Meanwhile, in the realm of high and medium density, high molecular weight film applications, industry is seeking resins with minimum extrusion line speeds of about 210 feet per minute and thus films with higher bubble strength and stability together with higher dart drop strength in the narrower gauges.